#define _CRT_SECURE_NO_WARNINGS 1

/**
 * Definition for singly-linked list.
 * struct ListNode {
 * int val;
 * ListNode *next;
 * ListNode() : val(0), next(nullptr) {}
 * ListNode(int x) : val(x), next(nullptr) {}
 * ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution
{
public:
	ListNode* swapPairs(ListNode* head)
	{
		if (head == nullptr || head->next == nullptr) return head;
		auto tmp = swapPairs(head->next->next);
		auto ret = head->next;
		head->next->next = head;
		head->next = tmp;
		return ret;
	}
};